Bias point control circuit for externally modulated transmitter

Abstract

Methods and apparatuses for controlling a bias point voltage for an external optical modulator are provided. A control loop is used to adjust a bias signal applied to an external modulator by determining unwanted signals at a predetermined frequency received by an optical receiver of the control loop and accounting for these unwanted signals when determining the bias point voltage.

The invention claimed is: 
     
       1. A bias voltage controller for an external optical modulator comprising:
 a signal generator configured to produce a first signal having a first frequency based on a first control input and a second signal having a second frequency; 
 a combiner configured to produce a bias voltage control signal based on the first signal and a bias point voltage; 
 an external optical modulator configured to operate at a bias point based on the bias voltage control signal and produce a modulated signal; 
 a detector configured to produce a first error signal based on the modulated signal and the second signal when the first control input is a first value and produce a second error signal based on the modulated signal and the second signal when the first control input is a second value; and 
 a processor configured to produce the bias point voltage based on the first error signal and second error signal. 
 
     
     
       2. The bias voltage controller of  claim 1  wherein:
 the first signal has a zero magnitude when the first control input is a first value and has a non-zero magnitude when the first control input is a second value; and 
 the bias point voltage is based on a difference between the first error signal and the second error signal. 
 
     
     
       3. The bias voltage controller of  claim 2  wherein the processor is configured to set the bias point voltage to a predetermined value if the difference between the first error signal and the second error signal exceeds a threshold value. 
     
     
       4. The bias voltage controller of  claim 2  wherein the processor is configured to maintain a previously computed bias point voltage if the difference between the first error signal and the second error signal exceeds a threshold value. 
     
     
       5. The bias voltage controller of  claim 1  wherein the second frequency is twice the first frequency. 
     
     
       6. A bias voltage controller for an external optical modulator comprising:
 a signal generator configured to produce a first signal having a first frequency based on a first control input and a second signal having a second frequency; 
 a combiner configured to produce a bias voltage control signal based on the first signal and a bias point voltage; 
 an external optical modulator configured to operate at a bias point based on the bias voltage control signal and configured to modulate an optical carrier with a third signal to produce a modulated signal; 
 an optical receiver to receive the modulated signal; 
 a switch configured to output the modulated signal from the optical receiver or a reference voltage when a second control input is a third value or fourth value respectively; 
 a detector configured to receive the switch output and the second signal wherein and produce a first, second, third, and fourth error signal based on the switch output and the second signal and a first, second, third, and fourth predetermined setting of the first and second control input, respectively; 
 a processor configured to produce the bias point voltage based on the first error signal and second error signal; and 
 a optical receiver monitor configured to compute the difference between the third error signal and the fourth error signal to determine whether the difference exceeds a threshold value. 
 
     
     
       7. The bias voltage controller of  claim 6  wherein:
 the first signal has a zero magnitude when the first control input is a first value and has a non-zero magnitude when the first control input is a second value. 
 
     
     
       8. The bias voltage controller of claim of  claim 7  wherein the detector is configured to produce a first error signal based on the switch output when the second control input is a third value and the first control input is a first value and the second signal and configured to produce a second error signal based on the switch output when the second control input is a third value and the first control input is a second value and the second signal and configured to produce a third error signal based on the switch output when the second control input is a fourth value and the first control input is a first value and the second signal and configured to produce a fourth error signal based on the switch output when the second control input is a third value and the first control input is a first value and the second signal. 
     
     
       9. The bias voltage controller of  claim 6  wherein the second frequency is twice the first frequency. 
     
     
       10. A method for controlling a bias point voltage for an external optical modulator, the method comprising:
 producing a first signal having a first frequency based on a first control input; 
 producing a second signal having a second frequency; 
 producing a bias voltage control signal based on the first signal and a bias point voltage; 
 receiving a modulated signal from an external optical modulator wherein the external optical modulator operates at a bias point based on the bias voltage control signal; 
 producing a first error signal based on the modulated signal and the second signal when the first control input is a first value; 
 producing a second error signal based on the modulated signal and the second signal when the first control input is a second value; and 
 producing the bias point voltage based on the first error signal and second error signal. 
 
     
     
       11. The method of  claim 10  wherein:
 the first signal has a zero magnitude when the first control input is a first value and has a non-zero magnitude when the first control input is a second value; and 
 the bias point voltage is based on a difference between the first error signal and the second error signal. 
 
     
     
       12. The method of  claim 11  further comprising setting the bias point voltage to a predetermined value if the difference between the first error signal and the second error signal exceeds a threshold value. 
     
     
       13. The method of  claim 10  wherein the second frequency is twice the first frequency. 
     
     
       14. The method of  claim 10  further comprising:
 producing a third error signal based on a reference voltage and the second signal; 
 producing a fourth error signal based on the modulated signal when the first control input is a first value and second signal; and 
 computing the difference between the first error signal and the second error signal.